Combine register lists of smp targets

[Tim N. <ti...@si...>]

I've had a change in review <https://review.openocd.org/c/openocd/+/6362>
since July:

> Combine register lists of smp targets.


This is helpful when you want to pretend to gdb that your heterogeneous
> multicore system is homogeneous, because gdb cannot handle heterogeneous
> systems. This won't always work, but works fine if e.g. one of the
> cores has an FPU while the other does not. (Specifically, HiFive
> Unleashed has 1 core with no FPU, plus 4 cores with an FPU.)


There was some discussion which didn't lead to a clear conclusion, and
nobody has responded to my comments there since September. Can somebody
take a look?

Thank you,
Tim